Field of the Invention
The invention concerns a method for determining a pulse sequence for radial sampling (data entry into) of k-space in the context of magnetic resonance imaging. The invention further relates to a magnetic resonance imaging method and a pulse sequence optimization computer. Furthermore, the invention relates to a magnetic resonance imaging apparatus.
Description of the Prior Art
Modern imaging methods are often used to generate two-dimensional or three-dimensional image data, which can be used to visualize an examination object that is being depicted, and as well as for further applications.
In a magnetic resonance installation, also known as a magnetic resonance tomography apparatus, the body to be examined is usually exposed to a relatively high basic magnetic field, e.g. 1, 3, 5 or 7 Tesla, by a basic field magnet in the scanner of the apparatus. In addition, a magnetic field gradient is applied by a gradient system of the scanner. Radio-frequency excitation signals (RF signals) are then radiated by a radio-frequency transmitting system using suitable antennas in order to tip the nuclear spins of specific atoms, which are resonantly excited by this radio-frequency field, by a defined flip angle relative to the magnetic field lines of the basic magnetic field. During the relaxation of the nuclear spins, radio-frequency signals (called magnetic resonance signals) are emitted and are received by suitable receiving antennas, and then processed further. The desired image data can then be reconstructed from the raw data acquired in this manner.
For a specific measurement, it is necessary to emit a specific pulse sequence in the scanner comprised of a series of radio-frequency pulses, in particular excitation pulses and refocusing pulses, and suitable gradient pulses, which are emitted in a coordinated manner, in various spatial directions. Readout windows must also be set in a temporally suitable manner, so as to specify time periods in which the induced magnetic resonance signals are captured. In this case, the imaging is influenced by the timing within the sequence, i.e. the temporal intervals at which the pulses follow each other. A number of control parameters are usually defined in a measurement protocol, which is created in advance and is retrieved for a specific measurement, e.g. from a storage entity, and may be modified on site by the operator. The operator can specify additional control parameters such as a specific slice spacing of a stack of slices to be measured, a slice thickness etc. A pulse sequence, also known as a measurement sequence, is then calculated on the basis of all these control parameters.
The gradient pulses are defined by their gradient amplitude, the time duration of the gradient pulse, and the steepness of the edge, or the first time derivative of the pulse waveform (dG/dt) of the gradient pulses, also referred to as the “slew rate”. A further important gradient pulse variable is the gradient pulse moment (also shortened to “moment”), which is defined by the integral of the amplitude over time.
The magnetic gradient coils by which the gradient pulses are emitted are switched frequently and rapidly during a pulse sequence. Since the time specifications within a pulse sequence are usually very strict and the total duration of a pulse sequence, which defines the total duration of an MRT examination, must also be kept as short as possible, gradient strengths of approximately 40 mT/m and slew rates of up to 200 mT/m/ms must be achieved in some cases. The strength of the gradients results in a higher energy consumption and also places higher demands on the gradient coils and the other hardware. Since the hardware has a maximum load limit, the gradient strength and the increase of the gradients (slew rate) are generally limited.
In the case of magnetic resonance imaging, direct image recording (acquisition) does not take place in the spatial domain, rather magnetic resonance signals are acquired that have an amplitude that can be transformed into the spatial domain by a Fourier transform of the image data entered into k-space. K-space is the positional frequency space of the density distribution of the magnetic moments in an examination region in which MR signals are recorded. Once k-space has been sufficiently sampled (filled with acquired data), a Fourier transformation (which is two-dimensional when sampling by slice) produces the spatial distribution of the density of the magnetic moments (image data). During the measurement (scan), k-space is filled with raw data that correspond to the acquired magnetic resonance signals. In this case, lines in a Cartesian grid of k-space are usually sampled consecutively. This generally has the advantage that any shifts in the measured lines are identical for every k-space line. This coherent shifting results in a phase offset of the image data. Since only the magnitudes of the image signals are considered in most image recordings, this phase offset no longer occurs in the representation in the spatial domain of the image data. Therefore, this type of k-space sampling is very robust.
K-space can also be filled with raw data using other sampling patterns (paths), called trajectories. For example, sampling of k-space can take place using a radial or spiral trajectory. These sampling patterns each have specific advantages and disadvantages.
Although conventionally most MR image recordings are made using Cartesian trajectories in the past, radial MR imaging is growing in importance. The advantage of MR imaging using a radial trajectory is its particularly robust nature with respect to artifacts caused by movement. This property is particularly relevant, for example, if an MR image recording is made in the chest region of a breathing patient. Radial MR imaging is also suitable for rapid image recordings since it responds robustly to undersampling, and therefore less k-space data need to be captured, without artifacts becoming noticeable in the image recording. However, radial MR imaging is vulnerable to possible shifts of the radial trajectories in k-space, e.g. resulting from minimal time delays.
In the context of MR imaging with radial k-space sampling, usually data for a different radially extending “spoke” through k-space are acquired with each repetition of the excitation/readout portion of the sequence, each spoke being tilted, e.g. by an angle Ω, relative to the physical axes of the magnetic resonance scanner. The physical axes of the magnetic resonance scanner correspond e.g. to the orientation of the coils of the gradient system. The longitudinal orientation of the basic magnetic field usually corresponds to the z-direction, the lateral direction corresponds to the x-direction, and the anterior-posterior direction corresponds to the y-direction.
For simplicity, an imaging method with slice-by-slice radial sampling of k-space (“stack of stars”) is described in the following. However, the following considerations also apply in principle to three-dimensional radial sampling of k-space.
In order to allow the MR signals in k-space to be read out in the radial direction, readout gradients Gr are generated in the radial direction with amplitudes GR, and phase gradients Gp are generated perpendicularly thereto in the phase direction, with amplitudes GP, during the readout of the MR signals. In this context, the direction of the readout gradients Gr and the direction of the phase gradients Gp are designated as logical axes. FIG. 1 shows a pulse sequence having such a pulse pattern, wherein the readout gradient Gr extends in the direction of the x-axis here and the phase gradient Gp is oriented in the direction of the y-axis. The x-axis and the y-axis are designated as physical axes in this context. Unlike the logical axes, the physical axes are fixed. FIG. 1 shows a snapshot of k-space sampling. Specifically, the readout gradient Gr corresponds in this situation to a gradient Gx in an x-direction with the amplitude Gx, and the phase gradient Gp corresponds in this situation to a gradient Gy in a y-direction with the amplitude Gy.
At the next repetition, the readout gradient Gr and the phase gradient Gp are rotated in each case by the angle Ω=Ωs, i.e. the angle Ωs between two k-space spokes or radial k-space trajectories, such that the amplitudes Gx, Gy, GR, GP of the gradients Gx, Gy, Gr, Gp are as follows:Gx=−sin Ωs·GR+cos Ωs·GP,  (1)Gy=cos Ωs·GR+sin Ωs·GP.  (2)
In order to ensure that the maximum permitted physical values of the gradients Gx, Gy in the x-direction and the y-direction, in particular their amplitude Gx, Gy and their increase dGx/dt, dGy/dt are not exceeded in any repetition, i.e. at any angle Ω, the values that are used in the entirety of the pulse sequence for the amplitude GR, Gp and the increase dGR/dt, dGp/dt of the logical gradients Gr, Gp are customarily limited such that the cited condition is satisfied at all times, i.e. for all angles Ω. This means that the amplitudes GR, Gp of the logical gradients Gr, Gp are selected such that the cited maximum values of the amplitudes Gx, Gy and the increases dGx/dt, dGy/dt are not exceeded at the angle Ωw, at which GP and GR are structurally superimposed on a physical axis. As a result of this global limitation relating to the gradient parameters of the logical gradients, minimal possible imaging parameters, e.g. the echo time TE and the repetition time TR, are significantly limited in a downwards direction, i.e. toward shorter times. In the case of many applications, particularly in the case of dynamic imaging or live imaging, it is desirable to set the values of the cited imaging parameters as low as possible.